1. Field of the Invention
This invention relates to automatic electro-optical devices for identifying fingerprints, and especially to devices of the type in which the incoherent optical image of a latent fingerprint is superimposed over a known fingerprint for comparison. More particularly, it contemplates a fingerprint identification device incorporating a method and means for rapidly sweeping an optically projected image of a latent print in a repetitive pattern across a surface containing a previously identified print and sensing the marked change in reflected or transmitted radiance which occurs when the features of identical prints coincide.
2. Prior Art
Once limited almost exclusively to the fields of criminal investigation and law enforcement, with the advent of automated fingerprint identification systems the use of fingerprints for personal indentification is now being extended to a broad range of applications. The ability to establish or confirm an individual's identity quickly and with near certainty readily lends itself to such purposes as access control, transaction approval in banking, mercantile and other commercial operations and record processing in the administration and delivery of health and welfare services.
For the most part, current fingerprint identification systems are of three basic types. One utilizes means such as a scanning electron beam to sense and encode the minutiae and other characteristics of the print to be identified. Converted to digital form, compressed and stored, the encoded data are processed by computor for comparison with similar data taken from previously identified prints. Devices of this type are expensive, and for a variety of reasons do not provide the accuracy, dependability, or utility required for the uses envisioned.
In the second type, the latent print to be identified is converted into a hologram, diffraction image, Fourier transform, or other analog representation, generally using coherent light techniques, for optical or digital correlation with a known print. These devices are no more accurate or reliable than those of the first type, and, at their present stage of development, most of them are far too sensitive to a variety of external influences to be of use in other than a laboratory environment.
The third type of identification system, exemplified by U.S. Pat. No. 3,928,842, employs a source of intense incoherent light and optical projection means, including an image forming lens system, to superimpose the image of a latent print on a known print. The radiance reflected or transmitted by the two fingerprint patterns is sensed to provide an indication of their correlation.
To insure the registration of any identical features which may reside in the two patterns, means are provided for sweeping the optically projected image of the latent print across the exemplary print in a repetitive raster. Typically, the image is deflected by a pair of mirrors positioned in the optical path and mounted for simultaneous limited oscillating rotation about mutually perpendicular axes. One mirror oscillates at a relatively slow rate to produce reciprocating movement of the image along a first axis in the plane of the print. The other moves at a much higher rate and produces reciprocating movement of the image along a second axis perpendicular to the first. The pattern traced by the image is designed to assure that if the prints are the same, their features will coincide at some point during the scan.
To compensate for possible angular misalignment of the two prints, independent means are provided for effectively rotating either the image or the entire sweep pattern with respect to the known print during the comparison cycle.
The optical autocorrelation technique on which such systems are based is generally considered to afford the highest degree of accuracy available with any type of pattern-matching system, and of the three types of systems mentioned, this one appears to have the greatest potential for widespread use.
Presently, however, devices of this type suffer from a number of disadvantages. Chief among these are their susceptability to a variety of influences which adversely affect their accuracy and reliability, their relatively slow speed of operation, and their considerable size. Of secondary, but nonetheless significant importance, they are costly to manufacture, and demand considerable maintenance. All of these deficiencies can be traced to the conventional prior art approach to fingerprint comparator design.
The ability of the optical autocorrelation method to discriminate between matching and non-matching fingerprints depends heavily on the sharpness of the image and its congruence with the exemplar print. Precise control over the focus and magnification of the image are critical. In accordance with prior art convention, as illustrated by U.S. Pat. No. 3,928,842, existing autocorrelation-type comparators are designed to employ a single optical lens system for this function. Previously there was no reason to consider using more than one lens system. Furthermore, the cost of the highly corrective optical elements required for such systems inveighed strongly against the use of a second set of lenses.
Practical considerations mandate that the one lens system used be as fast as practicable. Necessarily, such a system has very limited depth of field. As a consequence, even small variations in focal distance result in unacceptable changes in the clarity and size of the image. Prior art comparators are thus extremely sensitive to thermal deformation, mechanical displacement, optical misalignment, and any other factor which may affect the length of the image path.
In a focused image-scanning optical system, all of the light-deflecting elements used to generate the image scan pattern are required to be located between the lens and the image. Because of this requirement, another consequence of the use of only one optical lens system in the prior art comparator is that all of the image-sweeping components and their associated support structures and drive mechanisms must be positioned at one end of the lens system. Since there is only one stop in the optical system, no other arrangement is possible. This configuration results in the optical train taking up a considerable amount of space.
Additionally, because of structural restrictions imposed by the previously mentioned sensitivity of the overall system to variations in focal distance, the need to have all of the image-deflecting apparatus on one side of the lens system places severe limitations on the kinds of image-deflecting devices which can be employed for generating the sweep pattern and, in turn, on the operating speed of the comparator.
Heretofore, a resonant electromechanical torque drive, such as a conventional galvanometer, was considered to be the fastest device capable of deflecting the optically projected image without introducing substantial changes in the length of the image path, and corresponding changes in the focus and size of the image. Based on this belief, prior art comparators commonly employ a mirror mounted to a galvanometer to produce one of the two image-sweeping sweeping motions.
In using a torque drive, to maintain a fixed image path length the axis of rotation of the drive must be located as close as possible to the plane of the reflective surface. Although galvanometers capable of oscillating at very high rates are available, since the mass of the mirror and the supporting structure needed to satisfy this requirement are appreciable, in practice galvanometers producing on the order of only about 120 sweep cycles/second are used.
In light of the fact that the rapid sweep is the operational component of the optical system which effectively determines the time required for the image to complete one full sweep raster, the use of a relatively slow oscillating torque drive results in an undesirably long fingerprint comparison duty cycle. Fingerprint identification devices currently on the market generally require at least six seconds to make a positive identification. Clearly, faster image-deflecting means are called for.
One such means, the rotating multi-faceted mirror or "reflecting polygon", is well known. However, it does not lend itself to use in comparators of prior art design. Since the polygon's flat reflective surfaces travel circumfrentially around the polygon's axis of rotation, their passage through the optical path results in radical changes in the focal distance traversed by the image. Mirrors of this type have seen wide use, singly and in pairs, in projection systems for scanning lines or narrow, unfocused light beams, but I am aware of no application in which one, much less two of them has been employed in any system for projecting a focused image in a two-dimensional raster. I have devised such a system.